Simulcast broadcasting is a technique which has been proposed in the U.S. and other countries in order to provide digital television services (e.g., high definition television) without obsoleting analog receivers currently in use. Simulcast broadcasting refers to the simultaneous transmission of program material encoded in two different formats over respective television channels. Thus, for example, in the U.S., a particular program may be encoded in NTSC format for transmission over a first 6 MHZ television channel, and the same or different program may be encoded in a digital format for transmission over a second 6 MHZ television channel. Therefore, viewers equipped with NTSC receivers would be able to receive and reproduce the program encoded in the NTSC format by tuning to the first channel, while viewers equipped with digital receivers would be able to receive and reproduce the program encoded in the digital format by tuning to the second channel.
It is generally assumed that NTSC and digital programs will be allocated to different channels in any given service area. However, it is quite possible that an NTSC program transmitted in one service area will be allocated to the same channel as a digital program transmitted in a nearby service area. If so, the NTSC program and the digital program could interfere producing co-channel interference.
NTSC co-channel interference with a received digital signal is of particular concern due to the relatively large picture and sound carriers and color sub-carrier characterizing an NTSC transmission. NTSC co-channel interference could either reduce the performance of a digital receiver or render the digital receiver incapable of reproducing any viewable image at all.
U.S. Pat. No. 5,087,975 discloses various embodiments of a comb filter which are arranged to reduce NTSC co-channel interference in a digital receiver used in television broadcast system having 6 MHZ channels. For example, a comb filter is disclosed in FIG. 4B of this patent and includes a summer 144 having first and second inputs of the same polarity. The first input receives an input signal directly, and the second input receives the input signal through a delay circuit 142. The delay circuit 142 imposes a six symbol delay on the input signal in order to produce the filter response shown in FIG. 6 of this '975 patent. As can be seen from FIG. 6, two of the notches of the comb filter response substantially coincide with the picture carrier and the color subcarrier of the NTSC transmitted television signal in order to reduce NTSC co-channel interference.
As another example, a comb filter is disclosed in FIG. 13 of the '975 patent and includes a summer 202 having first and second inputs of different polarity. The first input receives an input signal directly, and the second input receives the input signal through a delay circuit 200. The delay circuit 200 imposes a twelve symbol delay on the input signal in order to produce the filter response shown in FIG. 15. As can be seen from FIG. 15, three of the notches of the comb filter response substantially coincide with the picture and sound carriers and the color subcarrier of the NTSC transmitted television signal in order to reduce NTSC co-channel interference.
This system operates very well to reduce NTSC co-channel interference. However, in a television system having 8 MHZ channels, such as the 8 MHZ channels used in a PAL television system, where co-channel interference arises in much the same way as NTSC co-channel interference does in the 6 MHZ NTSC television system, the system disclosed in the '975 patent operates poorly to reduce co-channel interference. This poor operation results because the picture and sound carriers and the color subcarrier of the analog television signal transmitted in the 8 MHZ channel have frequencies which are different than the frequencies of the picture and sound carriers and the color subcarrier of the 6 MHZ NTSC transmitted television signal. Therefore, the notches of the comb filters disclosed in the '975 patent are not appropriate for an 8 MHZ transmitted television signal.
In an effort to reduce co-channel interference in a digital receiver operating in an 8 MHZ television system, it has been suggested that the 8 MHZ digital channel be shifted by a suitable amount with respect to the corresponding 8 MHZ analog channel in an attempt to align the notches of the comb filter response with the picture and sound carriers and the color subcarrier of that corresponding 8 MHZ analog television signal. However, the notches of the comb filter response do not adequately align with the picture and sound carriers and the color subcarrier of the transmitted analog television signal by merely shifting the digital channel. Moreover, shifting of the digital channel results in the digital channel spilling over into the adjacent analog channel causing undue interference between the digital channel and the adjacent analog channel.
Accordingly, it has also been suggested to raise the symbol rate of the digital system transmitting into 8 MHZ channels. This symbol rate could be raised from the symbol rate of 10.76 MHZ for a 6 MHZ channel to a frequency as high as 14.34 MHZ for an 8 MHZ channel. Raising the symbol rate toward 14.34 MHZ has the effect of increasing the separation of the notches of the comb filter response. Raising the symbol rate from 10.76 MHZ, however, does not adequately align the notches of the comb filter response with the picture and sound carriers and the color subcarrier of the transmitted 8 MHZ analog television signal. Moreover, even a moderate increase in the symbol rate from 10.76 MHZ shifts the notch positions enough to require a shift of the digital signal downward in frequency in order to re-align the notches with the picture and sound carriers and the color subcarrier of the transmitted co-channel 8 MHZ analog television signal. This downward shift in the digital spectrum, however, results in a spill-over of the digital channel into the lower adjacent analog channel producing an undesired interference between the digital and lower adjacent analog channels that cannot be easily reduced.
In order to maintain the symbol rate as high as possible, it has further been suggested to provide the inputs of the summer 202 disclosed in the aforementioned '975 patent with the same polarity and to shift the digital channel. However, the notches of the comb filter response again cannot be made to align adequately with the picture and sound carriers and the color subcarrier of the transmitted 8 MHZ television signal.
The present invention is directed to an arrangement which adequately reduces co-channel interference from an 8 MHZ analog television signal in a digital television receiver.